feat(frame)!: Remove generic Backend parameter

[joshka]

The purpose of this change is to simplify UI code that uses the Frame
type. E.g.:

fn draw<B: Backend>(frame: &mut Frame<B>) {
    // ...
}

Frame was generic over Backend because it stored a reference to the
terminal in the field. Instead it now directly stores the viewport area
and current buffer. These are provided at creation time and are valid
for the duration of the frame.

BREAKING CHANGE: Frame is no longer generic over Backend. Code that
accepted a Frame will now need to accept a Frame.

[codecov]

Codecov Report

Merging #530 (93334b9) into main (d67fa2c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #530   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files          40       40           
  Lines       11267    11268    +1     
=======================================
+ Hits        10148    10149    +1     
  Misses       1119     1119           

Files Changed	Coverage Δ
src/layout.rs	98.38% <ø> (ø)
src/widgets/block.rs	93.44% <ø> (ø)
src/widgets/clear.rs	52.00% <ø> (ø)
src/widgets/scrollbar.rs	89.76% <ø> (ø)
src/terminal.rs	58.46% <100.00%> (+0.16%)	⬆️

[kdheepak]

I haven't checked all the tests but what happens if a terminal resize occurs? Does this handle that?

[joshka]

I haven't checked all the tests but what happens if a terminal resize occurs? Does this handle that?

Frames are only created by terminal.get_frame(&mut self) which borrows terminal mutably and prevents the resize method from being called. The viewport and buffer size can only be changed by something that can mutate the terminal. The following does not compile:

let backend = TestBackend::new(10, 10);
let mut terminal = Terminal::new(backend).unwrap();
let frame = terminal.get_frame();
terminal.resize(Rect::new(0, 0, 5, 5)).unwrap();

 1  error[E0499]: cannot borrow `terminal` as mutable more than once at a time
    --> src/terminal.rs:687:9
     |
 686 |         let frame = terminal.get_frame();
     |                     -------------------- first mutable borrow occurs here
 687 |         terminal.resize(Rect::new(0, 0, 5, 5)).unwrap();
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
 688 |         assert_eq!(frame.size(), Rect::new(0, 0, 10, 10));
     |                    ------------ first borrow later used here
 
 For more information about this error, try `rustc --explain E0499`.
 error: could not compile `ratatui` (lib test) due to previous error

[orhun]

Breaking but good. We should add "migration" instructions to the final commit message.

[joshka]

Breaking but good. We should add "migration" instructions to the final commit message.

Will do